The present invention relates to a functional material and a functional device, which particularly not only make an industrial revolution against conventional optical parts, but also are suitable for various applications to, typically, the industrial fields employing electromagnetic waves such as light and sound waves such as an ultrasonic wave.
In conventional optical devices such as an interference filter, since a stacked film is produced on the basis of a predetermined wavelength range of target transmission/reflection light, it is impossible to select the wavelength of transmission/reflection light from external after the production of the optical device. Even in a laser diode, since a material design is generally made on the basis of a predetermined wavelength of laser light to be emitted from the laser diode, it is impossible to select the wavelength of laser light from external after production of the laser diode. Meanwhile, there may be considered a method of simply selecting a wavelength of light by using a prism or the like; however, such a method has a large problem in terms of integration or the like because the direction of the optical path is changed depending on the wavelength of light and the refractive index of the material forming the prism.
On the other hand, recently, a new physical concept “photonic crystal” has appeared, and many research engineers have taken interests in such a photonic crystal and have come to be at grips therewith. It has been revealed by a theoretical approach of Ohtaka et al. that a medium, which has dielectric constants repeated at a periodicity (which is not necessarily large so much but may be as small as about five cycles) with a unit cycle on the order of a wavelength of a target electromagnetic wave, forms a physical concept “photonic band” similar to that of a band structure of electrons in a crystal (see documents (1) K. Ohtaka, Phys. Rev. B., 19(1979)5057-5067, (2) K. Ohtaka and Y. Tanabe, J. Phys. Soc. Jpn., 65(1996)2276-2284, (3) Kazuo Ohtaka, J. Phys. Soc. Jpn., 52(1997)328-335, and (4) H. Miyazaki and K. Ohtaka, Phys, Rev. B., 58(1998)6929-6937). Further, the fact that a “photonic band gap”, at which light having a specific wavelength is suppressed, appears in such a photonic crystal has been independently reported at the same time of year by Yablonovitch (see a document (5) E. Yablonovitch, Phys. Rev. Lett., 58(1987)2059-2062) and John (see a document (6) S. John, Phys. Rev. Lett., 58(1987)2486). In summary, it has been technically supported that the transmission of light having a specific wavelength through a medium can be suppressed by giving a desired periodicity to the medium, and the transmission of only light having a specific wavelength through the medium is allowed by inserting a disturbance in part of the periodicity of the medium.
Such a periodicity can be realized not only in the form of a one-dimensional structure such as a stacked film but also in the form a two-dimensional structure such as balls arranged on a plane or a three-dimensional structure such as balls or dice-like substances densely stacked in a box. A structure for disturbing the periodicity can also be freely inserted in a periodic structure in accordance with a desired design. It has been reported that the photonic crystal can freely reflects or wave-guides light (see documents (7) A. Mekis, J. C. Chen, I. Kurland, S. Fan, P. R. Villeneuve, and J. D. Joannopoulos, Phys. Rev. Lett., 77(1996)3787-3790, (8) J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, Nature, 386(1997)143-149, (9) S-Y. Lin, E. Chow, V. Hietala, P. R. Villeneuve, and J. D. Joannopoulos, Science, 282(1988)274-276). In this way, the photonic crystal has been extensively studied, for example, in the field of semiconductors (see a document (10) Toshihiko Baba, Michitaka Ikeda, Naohisa Kamizawa, Hans Blom, Appl. Phys. Vol. 67, 9(1998)1041-1045). In recent years, the photonic crystal has been in the spotlight and has been briefly introduced in other documents (see (11) Nikkei Electronics, 730(1998)57-63), (12) Osamu Izumi, Shoujiro Kawakami, Chemical Industry, the January number, (1999)47-52, and (13) Tetsuo Taniuchi, Metal, Vol. 68, 6(1998)26-34).
That is to say, it has been theoretically, experimentally found that the optical properties of a photonic crystal can be largely changed by inserting a foreign matter for disturbing the periodicity. Such a is photonic band structure is similar to an electronic band structure of a semiconductor in which a level is formed in a band gap by doping carriers in the semiconductor.
As will be described in detail later, unlike the conventional device aimed at a static effect obtained by making use of the periodicity of a dielectric substance on the order of a wavelength of light, a functional material and a functional device of the present invention are each aimed at a controllable dynamic effect obtained by inserting, in the functional material or functional device, a foreign matter (or material for disturbing the periodicity), a certain physical property of which is changeable on the basis of a signal supplied from external. The unique behavior of the “foreign matter” or “impurity” for disturbing the periodicity has been described, for example, in documents (14) Toyohiko Yatagai, Optics, Vol. 28, 1(1999)15-21, (15) K. Harada, K. Munakata, M. Itoh, N. Yoshikawa, H. Yonezu, S. Umegaki, and T. Yatagai, Jpn. J. Appl. Phys., 37(1998)4393-4396, and (16) Pioneering Research Promotion Project of Japan Society for the Promotion of Science: “Exploration of Next Generation Artificial Material”, special edition, (Proceedings of the First Open Symposium), Jun. 15-16, 1999, (at The Institute of Physical and Chemical Research), Pioneering Research Promotion Project of Japan Society for the Promotion of Science/News No. 5, (1999)60-67. Each of these documents, however, describes only a static device having static physical properties and does not describe any device exhibiting a controllable dynamic behavior.
In the current study situation for the photonic crystal, experiments have been made by using an orthodox static periodic structure, that is, a previously designed periodic structure only in order to further establish the theory of the photonic crystal, and no attempt has been made until now to dynamically change the characteristic of a periodic structure with an elapsed time. It should be noted that the technique described in the above document (16) uses a non-linear material as a foreign matter; however, such a non-linear material is not controlled from external.
To the best of the present inventor's knowledge, as patent documents regarding the “photonic crystal”, there have been disclosed five patent documents: Japanese Patent No. 2918881, and Japanese Patent Laid-open Nos. Hei 11-218627, Hei 10-284806, Hei 11-186657, and Hei 10-83005.
These documents, however, are little concerned with the present invention. Japanese Patent No. 2918881 is characterized by providing a resonator mirror used for laser oscillation, wherein the resonator mirror includes a multi-layer reflection film obtained by forming periodic oxidation states in a semiconductor active layer, and therefore, such a document is quite different from the present invention. Japanese Patent Laid-open No. Hei 11-218627 is characterized in that a triangular lattice-like refractive index changing (spherical) region is provided in a dielectric slab optical waveguide, and describes only part of the photonic crystal. The content associated with the photonic crystal described in this document, however, can be known to a person skilled in the art on the basis of a document (17) Kuniaki Nagayama, Surface, Vol. 31, 5(1993)353-360. Japanese Patent Laid-open No. Hei 11-218627 also does not examine the controllability of the refractive index changing region at all, and therefore, such a document is quite different from the present invention. Each of Japanese Patent Laid-open Nos. Hei 10-284806 and Hei 11-186657 is characterized in that a photonic crystal is provided outside a semiconductor laser, and therefore, such a document is quite different from the present invention. On the other hand, Japanese Patent Laid-open No. Hei 10-83005 has contents associated with the present invention. Hereinafter, a difference between each of the contents of Japanese Patent Laid-open No. Hei 10-83005 and the present invention will be described.
(1) Japanese Patent Laid-open No. Hei 10-83005 has sixteen claims. Of these claims, ten claims define “the wavelength of light corresponding to a photonic band end is set in the vicinity of a wavelength of transmission light”. On the other hand, the present invention is intended to obtain a new effect superior to that obtained by the content described in Japanese Patent Laid-open No. Hei 10-83005 by inserting “a foreign matter” for partially disturbing the periodicity in a periodic photonic crystal and giving “a kinetic function ability” or “a change in refractive index” to the foreign matter on the basis a signal supplied from external. As a result, according to the present invention, the wavelength of transmission light is not required to be positioned in the vicinity of the band end, but can be freely set depending on how to insert the foreign matter in the periodic photonic crystal.
(2) In Japanese Patent Laid-open No. Hei 10-83005, a resist line, an optical fiber core, a diffraction lattice, a phase-separation type block copolymer, or the like is used as a periodicity forming element, and a ultrasonic wave or the like is used as a tool for changing the periodicity. To use the diffraction lattice as the photonic crystal, however, light must be made incident on a medium on the surface of which irregular grooves are formed in such a manner that the incident light receives the effect of the periodicity of the irregular grooves, and more specifically, light must be made incident on the medium in parallel to the irregular grooves, with a result that most of the incident light passes through the medium without receiving the effect of the periodicity. The use of the diffraction lattice as the photonic crystal is thus poor in efficiency. The diffraction lattice basically supposes specific obliquely incident light; however, in the real situation, such a design fails to obtain a strict analytic solution like an interference filter (see a document (18) Toyonen Matsuda, Youichi Okuno, Optics, Vol. 27, 11(1998)626-631). Under such circumstances, if the diffraction lattice, which is used on the supposition of specific obliquely incident light, is applied to the photonic crystal, it is very difficult to estimate the physical meaning of such application. While there often appears the expression “a diffraction lattice for forming a photonic band” in Japanese Patent Laid-open No. Hei 10-83005, such description merely specifies a non-efficient optical arrangement.
In Japanese Patent Laid-open No. Hei 10-83005, a ultrasonic wave is used as a tool for changing the periodicity; however, the effect of using such a tool is similar to that of the known Raman-Nath scattering. The technique disclosed in Japanese Patent Laid-open No. Hei 10-83005 has a novelty in changing the present periodicity into another periodicity. On the contrary, the most important feature of the present invention lies in not changing all of the periodicity but changing part of the periodicity. In this regard, the present invention is quite different from Japanese Patent Laid-open No. Hei 10-83005.
(3) Japanese Patent Laid-open No. Hei 10-83005 describes five claims associated with “an optical functional device in which metal films are formed on front and back sides of a diffraction lattice”; however, each of the five claims has a limitation in using a diffraction lattice for forming a photonic band. The use of the diffraction lattice means the same non-efficient optical arrangement as described above in which light must be made incident on a medium, on the surface of which irregular grooves are formed, in parallel to the irregular grooves. On the contrary, the present invention is not limited to such a non-efficient optical arrangement and also not limited to the use of a diffraction lattice as a photonic crystal.
(4) In Japanese Patent Laid-open No. Hei 10-83005, a technique of interposing an electro-optic material (to which a voltage is applied from external) between diffraction lattices is described in the last claim; however, such a technique is also limited by the diffraction lattice, and therefore, the invention described in the last claim is quite different in optical arrangement from the present invention (see FIG. 15 in Japanese Patent Laid-open No. Hei 10-83005).
(5) In Japanese Patent Laid-open No. Hei 10-83005, there appears the expression “in consideration of a phenomenon that a group velocity dfÖ(k)/dk is reduced to be close to zero in the vicinity of a photonic band end” (paragraph number: 0021); however, such a phenomenon necessarily occurs at a boundary wavelength when a band gap is newly formed, which is apparent from the above-described documents (1) to (6).
The optical devices proposed as described above are each produced in accordance with a previously designed wavelength range of light, and therefore, in order to freely select a wavelength of light by external control, it is required to move, typically, the entire optical device. As a result, the speed of response of the optical device becomes significantly poor.
On the other hand, the application of an actuator/kinetic function device to optical parts is very limited, for example, to focal correction using a linear motor or movement such as rotation.
In view of the foregoing, it has been expected to develop a technique capable of largely changing a physical function of a functional device by a very small motion (the rate of motion is not necessarily small) whose dimension is different from that of the physical function to be changed, thereby realizing an artificial skin whose color tone is changeable, and largely contributing to the field of typically optical communication.